1. Field of the Invention
Embodiments of the invention described herein pertain to the field of multi-stage centrifugal pumps. More particularly, but not by way of limitation, one or more embodiments of the invention enable a centrifugal pump impeller support system and apparatus.
2. Description of the Related Art
Fluid, such as gas, oil or water, is often located in underground formations. In such situations, the fluid must be pumped to the surface so that it can be collected, separated, refined, distributed and/or sold. Centrifugal pumps are typically used in electric submersible pump applications for lifting well fluid to the surface. Centrifugal pumps impart energy to a fluid by accelerating the fluid through a rotating impeller paired with a stationary diffuser. A rotating shaft runs through the central hub of the impeller and diffuser. A motor upstream of the pump turns the shaft of the pump motor, and the impeller is keyed to the shaft, causing the impeller to rotate with the shaft.
Each rotating impeller and stationary diffuser pair is called a “stage”. The impeller's rotation confers angular momentum to the fluid passing through the pump. The changes in angular momentum convert kinetic energy into pressure, thereby raising the pressure on the fluid and lifting it to the surface. Multiple stages of impeller and diffuser pairs may be used to further increase the pressure lift. The stages are stacked in series around the pump's shaft, with each successive impeller sitting on a diffuser of the previous stage.
In both radial and mixed flow stages, one method of handling the axial thrust of the pump is to allow each impeller to move axially along the pump shaft between the diffusers. In such instances, the impeller is keyed to the shaft within a key groove that runs axially along the length of the shaft. When the impellers can move independently of the shaft, the pump is referred to as a “floater style” pump. When the impellers are not able to move independently of the shaft, the pump is referred to as a “compression style” pump.
Impellers typically have a “skirt” extending axially on the bottom side of the impeller. A downthrust washer extends radially along the bottom side of the impeller, adjacent to the bottom skirt. The bottom impeller skirt outer diameter (OD) rotates inside the diffuser exit inner diameter (ID). Typically, the diffuser exit includes a downthrust pad that is opposite the downthrust washer. FIG. 1A illustrates a conventional impeller and diffuser pair of the prior art. The close tolerance between conventional skirt OD 100 and conventional diffuser exit ID 105 provides a hydraulic seal when fluid is pumped. The hydraulic seal helps to maintain the lift produced from stage to stage. Conventional downthrust pad 115 traditionally carries the downthrust load of the pump, which conventional downthrust pad is in close tolerance with conventional washer 120. Conventional washer 120 is typically made of a phenolic resin material.
As the skirt wears, for example from abrasives such as sand, dirt, rock and other solid particles in the pumped fluid, the conventional gap 110 shown in FIG. 1B, between the conventional impeller skirt OD 100 and conventional diffuser exit ID 105 increases. As conventional gap 110 increases, more fluid and pressure leaks, and the pump performance is reduced. The conventional gap 110 (tolerance) between the impeller skirt OD 100 and diffuser exit ID 105 should be between about 0.012 inches and 0.016 inches diametrically (0.006-0.008 inches radially). Conventional gaps 110 in excess of about 0.016 inches diametrically cause reduced pump production, which may necessitate that the pump be pulled out of operation. Similarly, as conventional washer 120 breaks down due to abrasives in the pumped fluid, downthrust pad 115 becomes less effective in handling downthrust loads during pump operation. A similar tolerance and/or hydraulic seal on the top of the impeller is similarly susceptible to erosion from abrasives.
Conventionally, a hard coating such as tungsten carbide, has been applied to impeller skirts in order to prevent wear from abrasives in well fluid. However, coating an impeller is time consuming and expensive. Even if the impeller skirt is coated, conventional washer 120 is still susceptible to abrasive wear.
Separately, to extend downthrust protection, conventional centrifugal pumps employ a flanged sleeve, which is keyed to the pump shaft inside a stationary bushing. A conventional flanged sleeve and stationary bushing of the prior art is illustrated in FIG. 2. Conventional flanged sleeve 310 and conventional stationary bushing 300 form a bearing set which provides the pump with protection from downthrust. Conventional stationary bushing 300 is fixed into the conventional hub 315 of conventional diffuser 305. Conventional flanged sleeve 310 is keyed to the shaft of the pump and rotates at the same speed. The problem with conventional flanged sleeves and conventional bushings is that they fail to adequately protect hydraulic seal portions of the impeller from abrasive wear.
In addition to downthrust protection, centrifugal pumps require radial support to keep the shaft centered inside the impeller and diffuser stages. Without radial support the impellers can contact mating diffusers, creating excessive wear and leading to excessive vibration and potentially a pump failure. Conventional pumps rely on the impeller and diffuser hubs to provide radial support, and in some instances, tungsten carbide bearings at the very top and bottom of the pump stages. However, abrasive wear can reduce the radial support provided by the hubs, and in such instances, the top and bottom bearings may be insufficient to provide the required radial support throughout the pump.
As is apparent from the above, current centrifugal pumps support systems suffer from many shortcomings. Therefore, there is a need for a centrifugal pump impeller support system and apparatus.